The present invention relates to a power-servo booster, and more particularly, to a type thereof capable of providing urging force to a power piston which secures a diaphragm positioned within a shell by applying pressure differential to the diaphragm.
A conventional power servo booster of this type is shown in FIG. 1, wherein a shell 1 includes a front shell 2 and a rear shell 3 coupled to the front shell at its rear end. Within the shell 1, a power piston 4 is reciprocally provided, and an input shaft 5 and an output shaft 6 are connected to the power piston 4. A flexible diaphragm 7 has an inner peripheral surface 7a supported to the power piston 4 and an outer surface 7b secured to the coupling portion between the front and rear shells. The diaphragm 7 divides the interior space of the shell 1 into a negative pressure chamber 8 and an atmospheric pressure chamber 9. The negative pressure chamber 8 is communicated with a pipe 10 adapted to introduce negative pressure into the chamber 8.
According to the conventional power servo booster, thus constructed, during the non-operating state wherein negative pressure is introduced into the chamber 8 through the pipe 10 and no input is applied to the input shaft 5, the negative pressure is also introduced into the atmospheric pressure chamber 9 through a valve means (not shown) in the power piston 4, so that the power piston 4 is positioned close to the rear shell 3, as shown, by the biasing force of a restoration spring 11 positioned between the front shell 2 and the power piston 4. In this case, as shown in FIG. 1, the diaphragm 7 provides a folded portion 7c and a contacting portion 7d. The folded portion 7c extends from the outer peripheral end surface of the power piston 4 toward a front portion of the shell 1 (leftwardly in the drawing), and the contacting portion 7d contacts the inner surface of the shell 1 at the diaphragm portion between the folded portion 7c and the outer end 7b of the diaphragm. Maintaining this state, an input is applied to input shaft 5 by moving the same toward the front shell 2, fluid communication between the negative pressure chamber 8 and the atmospheric pressure chamber 9 is blocked by valve means, and simultaneously, atmospheric pressure is introduced into the atmospheric pressure chamber 9. As a result, pressure difference is provided between these chambers 8 and 9, to thereby provide urging force to the power piston 4, whereby the output shaft 6 is moved frontwardly with the aid of the pressure differential.
During air discharge working within a brake piping, after installation of such booster into an engine compartment of a vehicle, in case the input shaft 5 is immediately operated without the introduction of the negative pressure into the negative pressure chamber, a portion of the diaphragm 7 confronting the face (disc portion) of the power piston 4 may be spaced apart therefrom due to the inner pressure increase of the negative pressure chamber 8 upon frontward movement of the power piston 4. The diaphragm 7 may be arcuately deformed or curved rearwardly as shown in FIG. 2. Then if the power piston 4 is immediately shifted to its restoring position as shown in FIG. 1, the deformed portion of the diaphragm 7 adjacent to the rear face of the power piston may be folded or enrolled at the rear face of the power piston. As a result of this phenomenon, the output shaft 6 of the power piston 4 cannot be completely moved back to its original position. Since one end of the output shaft 6 is connected to a piston of a master cylinder (not shown), the piston of the master cylinder remains in its extended position and therefore sufficient air discharge is not completely attainable. The above-mentioned drawback (dashpot phenomena) is also described in detail in the commonly assigned U.S. Pat. No. 4,292,887.